This invention is generally related to cathode ray tubes for use in color television receivers. It is specifically directed to an improvement in a color cathode ray tube of the "post-mask-deflection" type described in U.S. Pat. No. 3,452,242 to Miyaoka. FIGS. 4 and 5 of that patent illustrate a color selection electrode consisting of parallel wires designated alternately 1a and 1b. All wires 1a form one group and all wires 1b form a second group. Following Miyaoka's description, a 280 volt difference in DC potential is maintained between the two groups, both of which are operated at approximately 20 kV with respect to the cathodes. As a consequence of this 280 volt difference, electron beams passing through the slots between the wires are deflected so that they overlap on the screen behind each wire of the more positively charged group, 1b in Miyaoka FIGS. 4 and 5.
The post-mask-deflection type of mask is an arrangement that makes it possible to use relatively wide slots between the wires, so that the percentage of electrons actually reaching the screen is 40-50%, compared to only 15-20% in a conventional color tube in which the color selection electrode is a simple shadow mask.
In spite of this advantage, that type of post-mask-deflection cathode ray tube has not found commercial use because of the need to insulate adjacent wires from each other so that the required potential difference can be maintained. This requirement made it impossible (prior to the present invention) to use a metal frame to support the shadow mask, such as that shown in U.S. Pat. No. 4,695,761 to Fendley, assigned to the assignee of the present invention.
U.S. Pat. No. 3,894,321, issued to Moore, and also assigned to the same assignee as this invention, discloses a method for processing a color cathode ray tube having a thin foil mask sealed directly to the bulb. A post-mask-deflection mask embodiment involving a high-transmission mask is shown in FIGS. 7 and 7a of the '321 patent, and is described in column 8, lines 14-40. Following the assembly process as described in the '321 patent, a foil mask 32 is laid over ledges 27 of the front panel 10. The excess foil which lies beyond the ledges 27 may then be cut away along dashed lines 34 and 36, thus leaving a set of tabs on each side of the panel.
To carry out the teaching of the '321 patent, the individual foil strips must be fastened under tension to ledges 27, which must provide insulation between adjacent strips. If ledges 27 are part of the front panel as contemplated by the '321 patent, insulation presents no problem, but to provide a bond strong enough to maintain the tension applied to the foil strips, solder glass (i.e., powdered low-melting-point glass) must be used. This requires tensioning the foil mask in a temporary stretching frame, placing frame and mask upon the front panel with ledges 27 solder glass-coated, passing the entire assembly through an oven where devitrification of the solder glass takes place, then cooling the assembly to room temperature, and finally cutting off the unneeded parts of the foil. Only at this point can the temporary stretching frame be removed to be used over again in connection with another panel. The devitrifying of the solder glass and cooling process typically takes several hours because the front panel must not be subjected to dangerous temperature gradients. Thus the process ties up expensive capital equipment, including the temporary stretching frame and space in the oven, for several hours.